Advanced semiconductor fabrication is employing increasing vertical circuit integration as designers continue to strive for circuit density maximization. Such typically includes multi-level metallization. Electrical interconnect techniques typically require electrical connection between metal layers or other conductive layers which are present at different elevations in the substrate. Such interconnecting is typically conducted, in part, by etching a contact opening through insulating material to the lower elevation metal layer or conductive region. Increased circuit density has resulted in narrower and deeper electrical contact openings between layers within the substrate. Adequate contact coverage within these deep and narrow contacts continues to challenge the designer in assuring adequate electrical connection between different elevation areas within the substrate.
As transistor active area dimensions approached one micron in diameter, conventional process parameters resulted in intolerable increased resistance between the active region or area and the conductive layer. The principal way of reducing such contact resistance is by formation of a metal silicide atop the active area prior to application of the conductive film for formation of the conductor runner. One common metal silicide material formed is TiSi.sub.x, where x is predominantly "2". The TiSix material is typically provided by first applying a thin layer of titanium atop the wafer which contacts the active areas within the contact openings. Thereafter, the wafer is subjected to a high temperature anneal. This causes the titanium to react with the silicon of the active area, thus forming the TiSi.sub.x. Such a process is said to be self-aligning, as the TiSi.sub.x is only formed where the titanium metal contacts the silicon active regions. The applied titanium film everywhere else overlies an insulative, and substantially non-reactive, doped or undoped SiO.sub.2 layer.
Ultimately, an electrically conductive contact filling material such as tungsten or aluminum would be provided for making electrical connection to the contact. However, tungsten adheres poorly to TiSi.sub.x. Additionally, it is desirable to prevent intermixing of the contact filling material with the silicide and underlying silicon. Accordingly, an intervening layer is typically provided to prevent the diffusion of the silicon and silicide with the plug filling metal, and to effectively adhere the plug filling metal to the underlying substrate. Such material is, accordingly, also electrically conductive and commonly referred to as a "barrier layer" due to the anti-difffusion properties.
One material of choice for use as a glue/diffusion barrier layer is titanium nitride. TiN is an attractive material as a contact diffusion barrier in silicon integrated circuits because it behaves as an impermeable barrier to silicon, and because the activation energy for the diffusion of other impurities is very high. TiN is also chemically thermodynamically very stable, and it exhibits typical low electrical resistivities of the transition metal carbides, borides, or nitrides.
TiN can be provided or formed on the substrate in one of the following manners: a) by evaporating Ti in an N.sub.2 ambient; b) reactively sputtering Ti in an Ar and N.sub.2 mixture; c) sputtering from a TiN target in an inert (Ar) ambient; d) sputter depositing Ti in an Ar ambient and converting it to TiN in a separate plasma or thermal nitridation step; or e) by low pressure chemical vapor deposition.
As device dimensions continue to shrink, adequate step coverage within the contact has become problematical with respect to certain deposition techniques. Chemical vapor deposition is known to deposit highly conformal layers, and would be preferable for this reason in depositing into deep, narrow contacts.
Organic compounds are commonly utilized as chemical vapor deposition precursors. One subclass of this group which is finding 10 increasing use in chemical vapor deposition of metals and metal compounds are organometallic precursors. Specifically, examples are the reaction of a titanium organometallic precursor of the formula Ti(N(CH.sub.3).sub.2).sub.4, named tetrakisdimethyl-amidotitanium (TDMAT), according to the following formulas: ##STR1##
Organometallic compounds contain a central or linking atom or ion (Ti in TDMAT) combined by coordinate bonds with a definite number of surrounding ligands, groups or molecules, at least one of which is organic (the (N(CH.sub.3).sub.2 groups in TDMAT). The central or linking atom as accepted within the art may not be a "metal" in the literal sense. As accepted within the art of organometallic compounds, the linking atom could be anything other than halogens, the noble gases, H, C, N, 0, P, S, Se, and Te.
The above and other chemical vapor deposition reactions involving organometallic are typically conducted at low pressures of less than 1 Torr. It is typically desirable in low pressure chemical vapor deposition processes to operate at as low a pressure as possible to assure complete evacuation of potentially undesirable reactive and contaminating components from the chamber. Even small amounts of these materials can result in a significant undesired increase in resistivity. For example, oxygen incorporation into the film before and after deposition results in higher resistivity. Additionally, it is believed that organic incorporation (specifically pure carbon or hydrocarbon incorporation) into the resultant film reduces density and resistivity. Such organic incorporation can result from carbon radicals from the organic portion of the precursor becoming incorporated into the film, as opposed to being expelled with the carrier gas. Carbon incorporation can also cause other undesired attributes in the deposited film, such as low density and poor long-term reliability.
A typical prior art construction and method and problems associated therewith is apparent from FIG. 1. There illustrated is a semiconductor wafer fragment 10 comprised of a bulk substrate 12 and a overlying electrically insulative silicon dioxide layer 14, such as borophosphosilicate glass (BPSG). Bulk substrate 12 includes a dopant diffusion/active region 16 to which electrical connection is to be made. A contact opening 18 is provided through BPSG layer 14 to electrically conductive active area 16.
A thin titanium layer 19 for silicide formation with substrate 12 is provided. A thin layer 20 of titanium nitride is- then deposited to within contact opening 18 to less than completely fill such opening. Titanium nitride layer 20 functions, at least in part, as an adhesion/nucleation layer for a subsequently deposited tungsten layer 22. Tungsten does not readily deposit over silicon dioxide, silicide or an exposed silicon substrate. Unfortunately, an undesired keyhole 24 typically forms leaving a void within contact opening 18. Subsequent planarize etching of layers 22 and 20 relative to insulating dielectric layer 14 can undesirably result in outward exposure or opening of such void.
Keyhole formation could substantially be avoided were contact opening 18 filled substantially entirely with titanium nitride, and a subsequently sputter deposited metal layer thereby not having to be utilized. However, titanium nitride has not heretofore been utilized to provide the substantial or predominate plugging material within a contact opening due to its inherent high diffusivity to oxygen upon mere exposure to an oxygen containing ambient. Oxygen incorporation into a titanium nitride film fundamentally lowers the film's conductivity. Further, less dense films typically contain more oxygen.
It would be desirable to overcome some of the above drawbacks in a manner which enables titanium nitride to be used as the predominate material which fills a contact opening for making electrical connection to a diffusion region in a semiconductive substrate. Although the invention arose in connection with -formation of such contact plugs, the artisan will appreciate applicability of forming interlevel interconnects to other substrate constructions, such as between two elevationally separated conductive lines or layers. Further, the invention also primarily arose out of concerns and problems developed in chemical vapor deposition of titanium nitride layers utilizing organic precursor compounds. Yet again, the artisan will appreciate applicability of the invention to other electrically conductive materials which are undesirably capable of absorbing oxygen, when exposed to an oxygen containing ambient, effective to render such materials less electrically conductive.